Audio Global Viewpoint – October 2018

Avoid These Common Mistakes When Using Wireless Microphones

Let’s be clear about one thing. Wireless microphones — while freeing users from the restrictions of cables — also can also create headaches. Making a wireless system behave predictably is a challenge faced by all — from touring professionals to first time users. No one and no wireless system made by anyone is immune from problems.

Learning the basics about how wireless systems function can save trouble. It can help users limit dropouts, interference and distortion. Shure, a long-time manufacturer of a range of wireless mic systems, offers some suggestions to avoiding common errors.

Shure BLX24R Wireless Mic Systems

Shure BLX24R Wireless Mic Systems

Signal blockage

Shure suggests users maintain line-of-sight between the transmitter and receiver antennas as much as possible. Avoid metal objects, walls and large numbers of people between the receiving antenna and its associated transmitter. Ideally, this means that receiving antennas should be in the same room as the transmitters and elevated above the audience or other obstructions.

Line of site is critical. The microphone transmitter must be able to 'see' the receive antenna. Click to enlarge.

Line of site is critical. The microphone transmitter must be able to 'see' the receive antenna. Click to enlarge.

The human body will absorb, block, interfere and reflect the RF signal emitted by a wireless mic transmitter. Largely composed of salty water, human bodies soak up RF energy. In addition, if a user cups his or her hands around the external antenna on a handheld transmitter, its effective output can be reduced by 50 percent or more. Similarly, if the flexible antenna on a body-pack transmitter is coiled or folded, the transmitted signal is severely reduced in strength.

Incorrect antenna type or placement

Receiver antennas are one of the most misunderstood areas of wireless microphone operation, said Shure. Mistakes in antenna selection, placement or cabling can cause short range, dead spots in the performance area or low signal strength at the receiver that leads to frequent dropouts.

Diversity receivers offer much better performance than single-antenna types, but the antennas must still be put in the proper locations to maximize the performance and reliability of the system. To ensure good diversity performance, space antennas apart by at least one-quarter of a wavelength (about five inches at 600 MHz). One wave length (about 20 inches at 600 MHz) is even better. The receiver antennas should be angled apart in a wide "V" configuration, which provides better pickup when the transmitter is moving around and being held at different angles.

Try to keep antennas as close to transmitters with line of sight as possible. Antennas can also be frequency band-specific. Don't try to use an antenna from another system without double-checking the frequencies first.

If the receiver will be located away from the performance area (in an equipment closet or a closed rack, for example), ½-wave antennas or directional antennas should be remotely mounted (ideally above the audience) in order to have a clear line of sight to the transmitters. (Short ¼-wave antennas should never be remotely mounted, however, because they need the receiver chassis as a ground plane.)

A diversity antenna system will improve wireless microphone performance. The antennas should be spaced apart by at least one-quarter of a wavelength (about five inches at 600 MHz). Click to enlarge.

A diversity antenna system will improve wireless microphone performance. The antennas should be spaced apart by at least one-quarter of a wavelength (about five inches at 600 MHz). Click to enlarge.

Increasing the separation between diversity antennas up to one wavelength (about 20 inches at 600 MHz) will improve diversity performance. Beyond one wavelength, extra distance between the antennas will not significantly improve diversity performance, but may allow better coverage of a large stage, church or meeting room.

If the antennas are far from the stage,use directional antennas to improve reception by picking up more signal from that direction and less from other angles. If the antennas will be connected to the receiver with a length of coaxial cable, in-line antenna amplifiers may be required to overcome the inherent signal loss in the cable. The amount of loss depends on the exact length and type of cable used, so follow the manufacturer's recommendations. Total net loss should not exceed five dB.

Poorly coordinated frequency set

A properly coordinated set of wireless frequencies must satisfy two criteria: the frequencies must avoid local active TV channels and they must be mutually compatible.

Television transmitters may operate at power levels up to one million watts while wireless microphone systems typically have only 50 mW (fifty one thousandths of one watt!) or less output power. To combat broadcast television interference, avoid using frequencies of local active TV channels.

How local is local? "Local" is generally considered to be up to 50 or 60 miles, depending on the coverage area of the particular TV transmitter and on the location of the wireless microphone system. The good news is that indoor setups are at less risk than outdoor setups because building structures will usually strongly attenuate TV signals.

Inside buildings of substantial construction, it may be possible to ignore TV stations as close as 30-40 miles. Still, since the locations and assignments of television stations are well known, it's pretty easy to choose relatively safe wireless microphone system frequencies in a particular area.

Typical high-end wireless microphone transmitters will provide multiple controls and displays that can help in setup and reception troubleshooting.

Typical high-end wireless microphone transmitters will provide multiple controls and displays that can help in setup and reception troubleshooting.

To insure a mutually compatible set of frequencies once the local TV channels have been taken into account, it is necessary to use one of two methods. The simpler method is to use the "Group" and "Channel" frequencies that are already programmed into the wireless systems. By using Channels that are all in the same Group, compatibility is guaranteed for small setups of like equipment. The appropriate Group and Channels can be determined from a link to the manufacturer's website or often by using the built-in "Scan" function on the receiver itself.

If the wireless setup is more complex, for example using wireless microphones and wireless in-ear monitors together, it may be necessary to use a frequency coordination computer program to insure compatibility. Wireless manufacturers can assist in these situations.

One frequency does not fit all. When touring, one consequence of the newly dense TV channel distribution in the U.S. is that it is not generally possible to use a given set of wireless microphone frequencies everywhere in the country. There is no such thing as "set and forget.”

Even if an audio system doesn't move from place to place, the radio environment can change unexpectedly. It's largely true that television stations remain constant, but if there are other wireless systems in the frequency band — whether it's multiple systems in your own location or interference from the coffeehouse down the street — wireless frequencies may need to be adjusted. What worked at a sound check may not be fail safe when the show begins. That's why frequency coordination is so important.

Poor battery management

Despite the fact that transmitter battery life is a top concern with wireless mics, users continue to try and cut operating costs by using inexpensive batteries. Most wireless manufacturers specify alkaline or lithium single-use batteries because their output voltage is very stable over the life of the battery. This is important because most transmitters will exhibit audible distortion or signal dropouts when supplied with low voltage.

Rechargeable batteries often seem like the ideal solution, but many rechargeable cells provide about 20 percent less voltage than a single-use battery — even when fully charged.

To combat battery problems, carefully compare the transmitter's voltage requirements with the battery's output voltage over time to make sure that the battery will last through a full performance. For nine-volt applications, there are lithium-ion types that work well, while Ni-Mh and Ni-Cad batteries may last only a couple of hours. For AA applications, Ni-Mh rechargeable batteries offer similar performance to single-use alkaline batteries.

Using rechargeable batteries is a great way to save money and landfills as long as they are effectively managed. Remove batteries from transmitters after each performance. This will keep half-dead batteries away the next time the equipment is used and will prevent an accidental leak from damaging a transmitter if it’s stored for an extended period of time.

Improper gain set-up

Setting the proper input gain, Shure said, is one of the most important adjustments on a wireless microphone system. Distortion may occur if the gain is set too high, while poor signal-to-noise may result if the gain is set too low. Most wireless systems have a gain control on the transmitter itself in the form of a switch, a pot or a programmable adjustment.

Adjusting gain on a wireless microphone, typically requires looking at the receiver for any overload or peak indications.

Adjusting gain on a wireless microphone, typically requires looking at the receiver for any overload or peak indications.

It may help to think of this gain control as serving the same function as the "trim" or "gain" adjustment on a mixer. Its purpose is to set the input sensitivity low enough to prevent input overload or "clipping" but high enough so that the signal level is well above the system noise floor.

Adjustment of the wireless transmitter gain is done in the same way as mixer input gain: set the gain control so that the loudest input signal just barely lights the overload or peak indicator. For a wireless system this indicator is usually on the receiver, so it is necessary to observe the receiver front panel while the performer is singing or playing.

If the peak indicator is flashing constantly, reduce the transmitter gain until it flashes only occasionally. If the indicator never flashes, increase the gain until it flashes just on the loudest signals.

Many wireless microphone systems have an output level control on the receiver. Since this control only affects the receiver output, it has no effect on improper gain adjustment in the transmitter. If distortion or poor signal-to-noise is occurring in the transmitter, it cannot be "fixed" by changing the receiver output level. Most leave this control at maximum. As long as the mixer input can accommodate this level, the overall system will exhibit the best possible dynamic range.

And finally, whenever depending on wireless microphones systems, cover your bet. Carry along a traditional wired mic just in case trouble arises and you can’t figure out how to fix it in time for the performance.

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